Many MR processes such as imaging (MRI) and spectroscopy (MRS) require homogeneous static fields. It is customary with MR equipment to map the static field to determine the distribution of the inhomogeneity and to attempt to correct this inhomogeneity with shim coils. When mapping the static field it is often necessary to determine and correct for phase jumps.
There are different methods presently used for mapping the static field distribution. Among the prior art methods for measuring the static field distribution is a method disclosed by Maudsley et al in a article entitled "Field Inhomogeneity Correction and Data Processing for Spectroscopic Imaging" published in the Magnetic Resonance Medical Journal, 1985, vol. 2, pp. 218-233. See also the article by Maudsley et al. entitled "Magnetic Field Measurement by NMR Imaging" which appeared in the J. Phys. E:Sci. Instrum., vol. 17, pp. 216-220. Another method for measuring the static field distribution in magnetic resonance systems is described in an article entitled "A New Method of Measuring Static Field Distribution Using Modified Fourier NMR Imaging" by K. Sekihara et al., published in the J. Phys. E:Sci. Instrum., vol. 18, 1985.
The methods for measuring the static field distribution described in the articles used phantoms and not actual subjects within the bore of the magnet. It is well known that placing a subject (patient) in the bore of the magnet drastically alters the magnetic field distribution. See, for example, an article entitled "Separation of True Fat and Water Images by Correcting Magnetic Field Inhomogeneity in Situ" by H. N. Yeung, et al. which appeared in the Journal of Radiology, 1986, vol. 159, pp. 783-786. It is becoming more standard to measure the static magnetic field inhomogeneity in situ, that is, with the subject positioned within the bore. The measurement and correction of the magnetic field inhomogeneity with a patient in the bore (in situ) is especially required for in vivo localized spectroscopy and for chemical shift imaging where highly homogeneous magnetic fields within the region of interest are not only desired but are required.
As explained in the last cited article the magnetic field inhomogeneity is mapped using standard imaging sequences with minor adjustments for the mapping. For example, a two-pulse and two-echo sequence is used with variations in the time between the 90 and the 180 degree pulses and with the echo time being varied to emphasize and measure the field inhomogeneity. Thus, in general, the inhomogeneity is derived by determining a phase map of a portion image of the human body in situ. In this determination the natural 3.5 parts per million (ppm) chemical shift between water and fat molecules is cancelled by assuring that the phase differences between the water and fat signals are equal to 2.pi. or integer multiples of 2.pi.. In other words pulse sequences used for measuring the phase angles are adjusted so that the 3.5 ppm shift corresponds to an integer number of cycles. To make the adjustment it is necessary to measure the local inhomogeneity induced by the body part being scanned; since, as is well known this local inhomogeneity often causes a phase shift greater than the 3.5 ppm chemical shift between the water and the fat molecules.
Another and unique method for obtaining magnetic field inhomogeneity maps in situ is taught in a patent application filed in Israel on Nov. 30, 1986 and receiving Ser. No. 80814 (assigned to the assignee of this invention).
An ubiquitous problem present with phase mapping of MR system is known as "phase wrapping". Phase wrapping occurs when the phase angle is greater than 2.pi. (or less than 0) radians. The measuring system only measure angles between 0.degree. and 360.degree. (2.pi. radians) and angles over 360.degree. are registered as the difference between the actual angle and 360 degrees. However, the inhomogeneity of the static field often exceeds 2.pi. radians.
In other words the measurement of the field inhomogeneity results in phase jumps when there are phase angles that are greater than 2.pi.. Since the systems only measure angles between zero and 2.pi. a resultant phase angle between zero and 2.pi. may actually be "wrapped around;" i.e., be an angle greater than 2.pi.. Determining if a measured angle is in the range of zero to 2.pi., or is greater than 2.pi., or 4.pi., or greater than 2N.pi. is herein called "phase unwrapping".
Algebraic manipulations of the phase map require continuity in the region of the signal; therefore, it is necessary to perform phase unwrapping in order, among other things, to correct for the field inhomogeneities when doing in vivo localized spectroscopy and/or chemical shift imaging.
Accordingly, scientists and especially scientists in the MR field have been seeking arrangements to provide phase data that does not suffer from phase wrapping. For example, MR scientists desire continuous maps (free of phase wrapping) of the magnetic field distribution. To date there is no known satisfactory arrangement for the determination and correction for phase jumps. Known attempts to correct for phase wrapping take too long and accordingly, cannot be performed while the patient remains substantially motionless in the bore of the magnet.